Marathon Running Can Damage the Heart — But Only Temporarily

As grueling — and potentially painful — as running a marathon can be, most people assume that completing one is a good thing overall, for your heart and your health.

That might keep some hopeful marathoners motivated on those endless training runs, but if you’re like me, you can’t help but wonder whether all the extra heart-pumping a long-distance event requires isn’t having some negative effect. After all, haven’t some marathoners had chest pain or even heart attacks during or after a race? (More on Time.com:Fitness Tech: 10 Cool Ways to Get in Shape)

Dr. Eric Larose, a cardiologist at the University Institute of Cardiology in Québec and a marathon runner himself, was curious about the disconnect as well. In a new study designed to document exactly what kind of stress that running a marathon put on the heart, he found to his surprise that it actually damages the muscle. The effect is temporary and reversible, but real nonetheless.

In a study presented at the Canadian Cardiovascular Congress 2010, Larose and his colleagues recruited 20 marathons runners for the study; some were training for the their first event, while others were making their 30th run. The volunteers were tested six to eight weeks before the marathon, immediately after they crossed the finish line, and then again three months later in order to create a complete picture of how a runner’s heart was affected by the race. (More on Time.com:Photos: Extreme Marathon Running)

The measurements were some of the most extensive done on such runners to date, says Larose. The tests included imaging studies of the heart to pinpoint exactly how the organ reacted to the strain of the marathon, as well as measures of inflammation and blood flow. The scientists also measured the subjects’ fitness levels by recording their changing respiration rate and gas exchange of oxygen and carbon dioxide. “To be honest, I was hoping we wouldn’t find any damage, or no permanent damage,” he says.

It turns out he was half right. The team documented many signs of a heart in distress, similar to the changes that might occur during a heart attack. In the runners, levels of an enzyme called troponin, which rises in response to a strained heart and reduced flow of blood, were highest immediately after the marathon. The pre-marathon imaging tests showed that the heart was functioning at below normal capacity, not pumping blood as efficiently and struggling to battle inflammation. All of these factors caused damage to the heart muscle. “There is temporary damage to the myocardium, not in all regions but in over 50% of the heart,” says Larose.

The damage was more pronounced in those who were less fit. “People who were less ready or less fit pushed the envelope more and induced more reversible injury to the heart,” he says. (More on Time.com:Top 10 Endurance Competitions)

While the results were sobering, the scientists were encouraged by the fact that at the three-month follow up, the damage induced by the marathon appeared to have dissipated, and the heart had resumed its normal functions. It’s still not clear whether the cumulative effect of running repeated marathons may eventually take a toll on the heart, but for now, the findings suggest that there may be a minimum fitness level needed beyond which the heart can bounce back from the strain of training and running a long race.

Larose is continuing his studies to determine whether using a fitness screen can help weekend road warriors decide whether they are ready to take on the challenge of a marathon. “Fitness matters,” he says, admitting that the study has changed his own training methods. “When I go out and run, if I’m not feeling good or more out of breath than usual, I won’t push the machine, now that I’m aware that I may be causing prejudice to my body and my heart,” he says.

The study hasn’t changed the basic message about getting as much physical activity as you can, he says. Just be smart about knowing when you may be pushing your body to do too much.